Obsessive-compulsive disorder (OCD) is a severe anxiety disorder characterized by unwanted and intrusive thoughts, images, or impulses and/or repetitive behavior. OCD affects 1-3% of the world's population and is a leading cause of illness-related disability. Recently, the first genome-wide association study (GWAS) of OCD found a SNP, rs6131295, that achieved genome-wide significance in the trio portion of the sample. The gene closest to rs6131295 is BTBD3, and variation at this SNP regulates expression levels of BTBD3. BTBD3 is a member of a large family of transcription factors including BTBD9, a gene associated with Tourette's Syndrome (TS), a disorder frequently comorbid with OCD. We examined BTBD3 knockout (KO) mice for phenotypes relevant to OCD. Rather than considering BTBD3 KO mice a model for OCD as a categorical disorder, we focused on several behavioral domains including exploratory behavior, repetitive/compulsive behavior, and sensorimotor gating, that are altered in OCD and other related disorders including TS and autism. We found that BTBD3 KO mice exhibit increases in repetitive/compulsive behaviors, and reductions in exploratory behavior and sensorimotor gating. Our findings that BTBD3 KO mice have deficits across these domains increase the probability that BTBD3 was a true hit in the OCD GWAS. Far fewer risk genes have been identified for OCD than for other neuropsychiatric disorders such as schizophrenia and bipolar disorder, so increasing the number of genes that predispose to OCD would be highly significant. This proposal will use a novel F1 screening strategy in mice to identify epistatic modifiers of BTBD3. We will cross inbred C57BL/6J mice that are heterozygous for a KO allele of BTBD3 with 33 different inbred strains to produce large a F1 cohort. Half of the F1s will be wild type (WT), and half will be heterozygotes (HT) at the BTBD3 locus. F1 mice will be efficiently phenotyped for the behavioral traits we found to be altered in BTBD3 HT mice, including exploratory behavior, repetitive/compulsive behavior, and sensorimotor gating. These data will reveal which F1 genetic backgrounds modify the behavioral effects of carrying the BTBD3 KO allele. We will then perform a murine GWAS to identify genetic modifiers of BTBD3. Minimal genotyping will be required, since all of the genome besides the BTBD3 locus will be predicted from existing SNP databases. The genetic modifiers of murine BTBD3 that we identify can then be tested in the human GWAS datasets that were used to identify rs6131295 as a risk allele for OCD, and large GWAS datasets for TS and autism. This novel approach should greatly improve the power to detect epistatic modifiers in human genetic datasets by prioritizing specific comparisons and drastically reducing the number of statistical tests performed. Our approach could provide a novel strategy for identifying epistatic modifiers. Identifying genes that interact with BTBD3 will further our understanding of OCD susceptibility and pathophysiology, and other disorders involving similar behavioral domains including TS and autism.